Wireless devices such as cellular telephones operate using a range of frequencies and need to maintain signal integrity across a frequency range. Increasingly there is a need for tunable antennas in wireless devices such as cellular telephones to maintain signal integrity over a range of frequencies, reduce power consumption and adjust to changing environmental and user conditions. Wireless devices such as cellular telephones have employed CMOS and MEMS tunable capacitors for digital tunable antenna applications, but to date, there has not been a widespread adoption of MEMS tunable capacitors that have contacting surfaces and may suffer from elements sticking together, dielectric charging, and changes in impedance and restoring force after repeated use. Various implementations of tunable MEMS inductors have been described in the prior art that use contacting surfaces but are subject to performance degradation due to changing impedance values, changing restoring forces and stiction from contacting surfaces. Accordingly, there is a need to provide tunable reactance in circuits with low loss and high quality factor that avoids the reliability problems with contacting surfaces.
In a tunable antenna application, it is desirable to tune a circuit toward resonance to increase signal gain. In a series RLC circuit, the resonance frequency is given by the formula:ω0=1/SQRT(LC)where L is the inductance and C is the capacitance in the circuit. Therefore, it is desirable to be able to tune both the inductance L and capacitance C of a tunable antenna circuit to provide a wider tuning range and allow more design flexibility.
The impedance Z of an element in an AC circuit is given byZ=R+jX where the real part of the impedance, R, is the resistance of the element, j is the square root of minus one, and the imaginary part of the impedance, X, is the reactance of the element due to capacitance and inductance. The inductive reactance of an element is ωL and the capacitive reactance of an element is 1/ωC where ω is the angular frequency of oscillation. There is a need for fast switching, repeatable tunable inductors and capacitors for tunable antenna applications.
This “Discussion of the Background” section is provided for background information only. The statements in this “Discussion of the Background” are not an admission that the subject matter disclosed in this “Discussion of the Background” section constitutes prior art to the present disclosure, and no part of this “Discussion of the Background” section may be used as an admission that any part of this application, including this “Discussion of the Background” section, constitutes prior art to the present disclosure.